Photolithography process is a key step of modern semiconductor fabrication, a strip width is a key parameter which characterizes the photolithography process, a real-time monitoring is needed when performing the photolithography process. One monitoring method is to form a unified testing graphic in a scribing slot by performing the photolithography process, and test the strip widths of these testing graphics. FIG. 1 is a schematic view showing a die 11 and a scribing slot 12 on a wafer, the scribing slot 12 is a white structure separating dies 11 in FIG. 1. Actually, the scribing slot 12 is not as wide as that shown in FIG. 1.
A unified testing graphic rule is provided in the scribing slot, special marks of several levels are provided, and they are easy to be recognized, automatic testing programs are developed for the testing. FIG. 2 shows two photolithography level monitoring graphics of the strip width of the scribing slot, English characters in the lower right corner are photolithography level marks, a strip-like structure is a testing graphic of the strip width. Generally, a single isolated line is tested, or one of dense strip-like structures is tested. The testing graphic of the strip width of the scribing slot can be applied to different products, which brings great conveniences for the real-time monitoring.
However, testing using a testing graphic of the scribing slot is deficient for some process in some level. For example, the product using a technology of local oxidation of silicon (LOCOS) has a great step in the die. For the polysilicon gate level in the LOCOS process, the testing graphic of the strip width of the scribing slot is generally formed on a silicon substrate by photolithography, the silicon substrate is flat. But the step would affect the polysilicon gate level on the die.
FIG. 3 is a top view of a typical device having a LOCOS structure, the device includes a source region 110, a field oxide region 120 and a polysilicon gate 130. FIG. 4 is a cross-sectional view of the device, FIG. 5 is a schematic view of the device when performing the photolithography process. According to FIG. 5, because of the step, vertical incident light will reflect on the other direction, it would lead to an exposure of a part of a photoresist 140 which should be blocked by a mask 20.
FIG. 6 is a photo of the product when performing a photolithography process, as we can see from the photo, because of the light reflected at the step, a local area of a photoresist graphic would become thin and slim. Under the extreme condition, a photoresist strip will break, which will affect a morphology and a strip width of an etched polysilicon gate. Such affection will be enlarged when a focal length of a photolithography machine is shifted.
As mentioned above, the testing graphic of the strip width of the scribing slot is a flat source region having a large area, which cannot reflect a situation of an actual strip width of a step structure in the die and will lead to that the die having an abnormal polysilicon gate will enter into a next process, a yield of the product will be affected.